1. Technical Field
The present invention relates to a semiconductor sensor with a diaphragm (a sensitive region of a semiconductor thin film) and a method of manufacturing the semiconductor sensor.
2. Related Art
Small-sized pressure sensors, vibration sensors and the like using semiconductor substrates have diaphragms for detecting pressures and vibrations. Methods of manufacturing diaphragms in semiconductor substrates include a method using a silicon on insulator (SOI) wafer in which a Si substrate and a thin film layer are bonded with an insulating layer interposed therebetween as disclosed in Japanese Patent No. 3506932 and Japanese Unexamined Patent Publication No. 2002-208708.
For example, in a semiconductor pressure sensor disclosed in Japanese Patent No. 3506932, which is shown in FIG. 1, an SOI wafer in which an insulating layer 12 (SiO2 layer) and a Si thin film layer 13 (Si layer) are laminated on a Si substrate 11 (Si layer) is used. The Si substrate 11 and the insulating layer 12 are partially etched from a bottom surface of the Si substrate 11 to form a diaphragm 15 made of the Si thin film layer 13 on a cavity 14, and strain gauges 16 are provided at edges of the diaphragm 15.
Further, in the semiconductor pressure sensor of Japanese Patent No. 3506932, a bottom surface of the diaphragm 15 is etched to the depth of about 5 to 10 μm in the cavity 14 by wet etching to dig a depression 17 on the bottom surface of the diaphragm 15.
The reason why the bottom surface of the diaphragm 15 is etched in the pressure sensor of Japanese Patent No. 3506932 is as follows. One of conventional pressure sensors leaves an insulating layer intact without etching, thereby forming a diaphragm of a double-layer structure made up of the insulating layer and a Si thin film layer. However, in such a pressure sensor, the diaphragm is distorted to have temperature characteristics because of a difference in coefficient of thermal expansion between the insulating layer and the Si thin film layer. The deformation of the diaphragm causes cracking to occur in the insulating layer. There is therefore a possibility that cracking extends to the Si thin film layer to break the diaphragm. Consequently, in a pressure sensor of Japanese Patent No. 3506932, the insulating layer 12 in the cavity 14 is removed, and further the bottom surface of the diaphragm 15 is also etched such that the depression 17 is dug down. Thereby the insulating layer 12 is completely removed.
Likewise, in a pressure sensor of Japanese Unexamined Patent Publication No. 2002-208708, a Si substrate and an insulating layer are removed by partial etching from a bottom surface of an SOI wafer to form a diaphragm. Further, the bottom surface of the diaphragm is etched such that a depression is dug down.